The SR-71 Pratt & Whitney JT11D-20B J58 Engine

The SR-71 Blackbird is powered by two Pratt & Whitney J-58 turbo-ramjets, each developing 32,500 pounds of thrust with afterburning. The critical problems concerning supersonic flight with air breathing engines are concentrated in the air inlet area. The circular air intakes of the SR-71 contain a center body tipped with a conical spike. The spike is movable, forward for takeoff and climb to 30,000 feet after which, as speed builds up, it moves rearward, controlling the amount of air entering the engine. As it does so, Air Inlet Bypass Doors in the side of the nacelle close to establish the correct flow of air through the engine, holding the supersonic shock wave in it's critical position within the inlet. The engine itself operates at subsonic speed. At Mach 3+ the spike is three feet to the rear of it's takeoff position, slowing down the incoming airflow, establishing an area of pressure within the nacelle, which is now pushing the engine. This action is so powerful that it accounts for 58 percent of the total thrust, the engine providing only 17 percent, and the ejectors (surrounding the nacelle near the afterburner) is responsible for the remaining 25 percent. Should the shockwave be expelled from the inlet, a condition known as an "Unstart" occurs. Unstarts have been known to be so violent as to crack the pilots helmet from the severe yaw of the aircraft. If unchecked, the resulting yaw is described by SR-71 pilots as though the nose and tail are trying to swap ends. However, an automatic control system senses this problem and repositions the Spike in milliseconds, doing so with great accuracy even though air loads of up to fourteen tons are acting on the spike, dealing with the difficulty before the human brain becomes aware of the problem, and the Blackbird cruises on....faster than a rifle bullet.

The first J-58s delivered to the blackbird program, all three models, had all stainless steel lines and the oil tank gold plated, the reason was for better heat dissipation. After a couple of years, and the subsequent tear down of engines, it was noted there was an abnormal amount of corrosion caused by dissimilar metal electrolysis. The gold plate was removed because the heat dissipation properties did not out weigh the cost of replacing lines as they started leaking.
Side note: When #957 crashed off the North end of the runway at Beale AFB and pictures were published, the hew and cry that came from the civilian segment about all the gold that was on the engine caused quite a commotion, even when it was explained why the gold was there. (Info courtesy Ron De Lozier)

Fri, 19 Jul 2002 08:18 Philippe Ricco Writes: 15 years ago, when I was a student and very impressed by the beautiful Blackbird, I wrote some personal notes about the Pratt & Whitney J-58. Years after, as Aeronautics Engineer, I found some more information to add to these notes.

Click on Picture to enlarge

The SR-71 aircraft, built by Lockheed, is a long-range, two-place, twin-engine airplane capable of cruising at speeds up to Mach 3.2 and altitudes over 85,000 ft (26,000 m). The aircraft is characterized by its black paint scheme; long, slender body; large delta wing; and prominent, spiked engine nacelles located midway out on each wing. The propulsion system of the SR-71 aircraft has three primary components. These components are axisymmetric mixed compression inlets, Pratt & Whitney J58 turbojet engines, and airframe-mounted, convergent-divergent blow-in door ejector nozzles.

The J58 engine was developed in the late 1950s by Pratt and Whitney Aircraft Division of United Aircraft Corporation to meet a U.S. Navy requirement. It was designed to operate for extended speeds of Mach 3.0+ and at altitudes of more than 80,000 ft. The J58 was the first engine designed to operate for extended periods using its afterburner, and it was the first engine to be flight-qualified at Mach 3 for the Air Force. The J58 was only used on the Lockheed YF-12 interceptor and its descendents, the A-12 and SR-71.

The inlet spike translates longitudinally, depending on Mach number, and controls the throat area. The spike provides efficient and stable inlet shock structure throughout the Mach range. At the design cruise speed, most of the net propulsive force derives from flow compression pressure on the forward facing surfaces of the spike. Besides the spike, other inlet controls include the forward and aft bypass doors, used to maintain terminal shock position and to remove excess air from the inlet; and cowl and spike bleeds, used to control boundary layer growth.

The SR-71 aircraft is powered by two 34,000 lbf (151,240 N) thrust-class J58 afterburning turbojet engines. Each engine contains a nine-stage compressor driven by a two-stage turbine. The main burner uses an eight-can combustor. The afterburner is fully modulating. The primary nozzle area is variable. Above Mach 2.2, some of the airflow is bled from the fourth stage of the compressor and dumped into the augmenter inlet through six bleed-bypass tubes, circumventing the core of the engine and transitioning the propulsive cycle from a pure turbojet to a turbo-ramjet. At Mach 3.2 cruise the inlet system itself actually provided 80 percent of the thrust and the engine only 20 percent, making the J58 in reality a turbo-ramjet engine. The engine is hydro mechanically controlled and burns a special low volatility jet fuel mixture known as JP7. The inlet bleed and aft bypass flow mix with engine exhaust flow just forward of the airframe-mounted ejector nozzle. Blow-in doors on the ejector nozzle remain open at low speeds and entrain additional mass flow into the exhaust stream. At high speeds, the doors close and the airframe nozzle ejector flaps reposition to form a convergent-divergent geometry. The blow-in doors and ejector flaps are positioned by aerodynamic forces.

The engine spikes and forward bypass doors are positioned by commands from the digital automatic flight and inlet control system (DAFICS). The DAFICS provides precise control of the terminal hock position. The DAFICS has significantly improved vehicle performance and range and has virtually eliminated inlet unstart, compared to the older analog control system.

A structurally modified SR-71 aircraft can carry external payloads weighing up to 20,000 lbm (9072 kg). This large weight limit permits flexibility in the configuration of a research package. However, within this weight limit, it is easy to design an external payload package whose additional drag exceeds the excess thrust capability of an SR-71 aircraft using unmodified J58 engines. To provide supplemental SR-71 acceleration, methods have been developed that could increase the thrust of the J58 turbojet engines. These methods include temperature and speed increases and augmenter nitrous oxide injection. The thrust-enhanced engines would allow the SR-71 aircraft to carry higher drag research platforms than it could without enhancement.

At maximum output the fuel flow rate in the J58 is about 8,000 gallons per hour and the exhaust-gas temperature is around 3,400 degrees. The J58 required the use of a special AG330 engine starter cart to spool the engines up to the proper rotational speed for starting. The cart was powered by two unmuffled Buick Wildcat V-8 racing car engines which delivered a combined 600 horsepower through a common gear box to the starter drive shaft of the aircraft engines. The J58s had to be spun up to about 3,200 RPM for starting.

At the speeds the SR-71 operated, surface temperatures were extremely high due to aerodynamic heating: 800 degrees at the nose, 1,200 degrees on the engine cowlings, 620 degrees on the cockpit windshield. Because of the operating altitudes, speeds, and temperatures, Lockheed designers were forced to work at the cutting edge of existing aerospace technology, and well beyond in many cases. Many features and systems simply had to be invented as they were needed, since conventional technology was inadequate to the task. New oils, hydraulic fluids, sealants, and insulations were created to cope with the ultra-high temperatures the craft would encounter. A new type of aviation fuel, JP-7, was invented that would not "cook off" at high operating temperatures, having such a low volatility and high flash point that it required the use of triethylborane as a chemical igniter in order for combustion to take place. The fuel itself was rendered inert by the infusion of nitrogen and then circulated around various components within the airframe as a coolant before being routed into the J58 engines for burning.

The B-58C was proposed as a lower cost alternative to the North American XB-70 or as a medium bomber to fill the gap between the XB-70 and the XF-108 Rapier Mach 3 fighter (proposal). The B-58C, or BJ-58, was proposed as a enlarged version of the B-58A to be powered by Pratt & Whitney J58 turbojet engines. The 32,500 thrust J58 was the same engine used on the Lockheed SR-71. Design studies were conducted with two and four engine designs. As enemy defenses against high speed, high altitude penetration bombers improved, the value of the B-58C diminished and the program was canceled in early 1961.

Global Security

The evening of Thursday, September 12, 2002 was probably the last time a Pratt & Whitney J58 will fill the night sky at Edwards with noise and light.

To experience a J-58 in full burner close up and personal is hard to describe. Picture a gigantic blow torch, 40 inches in diameter, putting out a blue-yellow-orange flame over 50 feet long. Imagine standing 30 feet from this, feeling the vibration and heat. You wear both foam plugs and earmuffs. Your ears still ring afterward, because the sound is conducted through your body. The back half of the engine transforms from dull gray to bright orange, seemingly transparent. The flame has little three-dimensional diamond shaped shock patterns about every two feet. I lost count at 13. It is both frightening and beautiful, an amazing demonstration of perfectly controlled power. And to think - this was done with 1950's technology.

Two J58s powered the SR-71 Blackbird. Individually, they have more horsepower than the Queen Mary. On a typical flight at Mach 3.2 and 80,000 feet, two engines would burn in excess of 100,000 pounds of fuel in a little over one hour.

NASA was the last organization to operate SR-71s. With the end of any flying of the aircraft, NASA was slowly disposing of the associated assets. Examples can be found in various museums around the country, having had their wings cut off for transportation and then tacked back on for display.

NASA still had something like 40-50 engines, most of them in flyable storage condition. One NASA center asked for three of the engines for testing purposes, so we had to prepare them for shipment and ensure that they were functioning. We also had promised the base commander that we would dispose of the remaining stock of JP-7, the exotic fuel that was specially formulated to withstand both the very cold environment at 80,000 feet and the very hot environment of the engine nacelle.

The best way to dispose of this fuel is to -- BURN IT. We also had to ensure that the triethylborane (TEB) was purged from the engine. TEB, which ignites upon contact with air, is used to start the engine and light the afterburners. Each engine carries enough TEB for any combination of at least 16 starts or lights.

Amazingly, NASA was able to assemble a team that still knew how to do this - most of them were still working for Pratt & Whitney at Edwards AFB. The former top sergeant of the detachment that worked the SRs for most of his AF career worked for NASA. Also amazing is that of the four engines removed from their shipping containers, three worked liked the day they were made (the forth had a broken line).

After the run, everyone stayed for cake donated by the P&W folks. The guys who ran the test stand posed for photos in front of the engine. There were actually some tears shed. These guys loved that program!

Wikipedia

References